Ceramic fiber reinforced filters designed for high temperature operation in fluid streams are known and have been described, for example in U.S. Pat. Nos. 4,761,323 to Muhlratzer, et al; 5,075,160 to Stinton, et al; 4,968,467 to Zievers and 5,196,120 to White.
Although varied in construction, the filters described in these patents listed above and filters made by others are primarily porous supports having particles or fibers interspersed throughout out the body of the support to control the ultimate porosity and average pore size of the filter's internal structure.
Filters having this type of construction allow the fluid to flow through removing particulate material from the fluid stream by trapping the particles that are too large to pass through in the pores of the internal structure of the filter. During the early stages of operation, a thin layer of particles separated from the fluid builds up on the surface of these filters, and the pressure drop across the filter increases requiring that the filter be cleaned or replaced in order for the fluid flows to be maintained at some acceptable operating conditions.
Generally these filters can be cleaned by a pulse of back pressure, but even with a back pressure pulse not all the trapped particles can be effectively removed since particles are not only on the surface of the filter, but are also trapped throughout out the internal structure. As the number of filtration/cleaning cycles increases, more particles becomes lodged within the internal structure of the filter leading to the phenomena termed blinding. As the filter continues to be cycled, particles become permanently lodged within the internal structure. Since to properly clean these filters trapped particles must travel from inside the filter to the outside walls of the structure without being trapped within the pores that are ever growing smaller from the clogs of other trapped particles, the initial pressure drop after cleaning continues to increase and the filter is eventually totally blinded and riseless.
To be desirable, a filter shoulder have long run cycles before cleaning is required, and the filter needs to be one which is easily cleaned. Successful cleaning is required to restore the filter to original operating pressures and to realize desirable service life. The consequence of being unable to remove essentially all the material blocking the pores of the filter surface and its internal structure is shorter and shorter operation cycles between filter cleaning cycles, short filter service life and frequent replacements filter and down times in units of the operating process.
The present invention is directed to a filter construction characterized by thin filtration layer which is deposited on the surface of a porous support. The filtration layer can be made to have an average pore size that much smaller than the particles that are to be removed from the fluid so that the layer essentially becomes a barrier to absolutely partition particles from the fluid flow without allowing these same particles to enter into and clog the pores of the filtration layer and of the support. Thus, the filter of the present invention is characterized by a low pressure drop across the filter in fluid flows, and a filtration layer that can be efficiently and easily cleaned by means of back pressure pulse through many operation cycles.